1. Field of the Invention
The present invention relates to the control of the movement of a person through a fluid of variable pressure, and in particular to systems for enabling a diver to ascend through a body of water at safe rate which enables him to avoid undergoing decompression sickness. The invention further relates to information systems for providing a diver with data involving a dive.
2. Description of the Prior Art
Air is a mixture of many gases, and is generally composed of 78 percent nitrogen and 20 percent oxygen. Under normal atmospheric conditions, a person breathes in air at about 14.7 p.s.i. A portion of the inhaled air passes through the lung tissues into the blood, and the blood supplies the cells of the body with oxygen from the air. The blood in turn receives carbon dioxide from the cells and discharges this waste gas back through the lung tissues to be exhaled. When an air supply is to be provided to a person in an environment which has a pressure substantially higher than normal atmospheric pressure, such as to a diver in a body of water, the air must be supplied under pressure to offset the ambient pressure. The apparatus for supplying pressurized air to a diver is referred to as self-contained underwater breathing apparatus or scuba equipment. This equipment has made possible extended dives to great depths, but the use of scuba gear is potentially very hazardous. The excessive breathing of pressurized nitrogen can cause narcosis (a state of stupor), although the presence of dissolved nitrogen has not generally been found to be injurious to the body tissues. However, should such dissolved gases be released from solution at some location where the gas cannot be discharged from the body through the lungs, decompression sickness or the bends occurs.
If the ambient pressure of a person decreases, such as during the ascent of a diver, inert (e.g. nitrogen) gases previously dissolved in the blood and other body tissues tend to be released from solution. If the body is able to discharge the body occurs. If the body is not able to discharge the nitrogen gas which has been released from solution, nitrogen bubbles form as a result of the supersaturated condition of the tissues in which they are located. These bubbles travel with the blood stream and, should they lodge in the heart or brain, can cause death or paralysis. It is therefore crucial that the body have sufficient time to discharge through he lungs any gases so released. The foregoing normally only aplies to the nitrogen from the air which has been inhaled, since oxygen in the blood passes to the body cells rather than remaining in the blood.
The dangers associated with ascents through a body of water are well known. More specifically, the importance of ascending according to a schedule which enables the discharge of nitrogen gas from the body is basic knowledge to the experienced diver. Such a schedule assures an ascent slow enough to avoid the release of nitrogen bubbles in the blood stream, and can call for various decompression stops in the course of the ascent to effect the necessary discharge of nitrogen through the lungs as the ambient pressure is reduced. The United States Navy developed and published the standard Decompression Tables, which set forth the depth and duration of decompression stops required to avoid decompression sickness according to the depth and duration of a person's dive. These tables are a basic tool of divers and are based upon an ascent rate of 60 feet per minute. Adherence to this ascent rate is essential if an ascent is to be made according to the U.S. Navy Decompression Tables.
Divers following an ascent schedule of the foregoing tables must carry with them during a dive a depth gauge and a watch, and they must carefully compute their ascent rate to maintain the 60 feet per minute ascent rate, in addition to making the prescribed decompression stops. The ascent rate computation is difficult to make and is often unreliable, and divers conventionally attempt to approximate the proper ascent rate by ascending at the same rate as their exhaled bubbles rise towards the surface. These efforts have been seriously deficient because the bubble ascent rate is difficult to discern, especially at greater depths wher it is most important. Furthermore, the diver may become disoriented in his ascent, and could ascend in a non-vertical directin or could even descend. The foregoing problems are increased as the diver's air supply becomes low, and he must rise quickly.
There have been attempts to develop alternatives to the U.S. Navy Decompression Tables. For example, systems have been proposed for simultating the flow of nitrogen through the body tissues of a diver to indicate to the diver a safe ascent schedule. Indicative of such devices are those disclosed in the following U.S. Pat. Nos. 3,121,333, 3,710,625, 3,757,586, 3,759,108, 3,759,109, and 3,457,393. U.S. Pat. No. 3,681,585 discloses an analog computer for indicating a diver's decompression schedule, and incorporates a plurality of resistor-capacitor sets serially connected in an electrical cascading relationship. During the diver's ascent, the capacitors discharge in a manner such that the voltage remaining across the undischarged capacitors indicates the proper ascent schedule. Such devices have not been generally accepted because of their expense, bulk, and lack of reliability.